feat(config): expand per-project overlay to cover provider, sandbox, approval, mcp_path, max_subagents, allow_shell (#485)

The project-config overlay (`<workspace>/.deepseek/config.toml` merged
on top of the user's global `~/.deepseek/config.toml`) was already
wired but only carried four string fields: model, api_key, base_url,
reasoning_effort. The use cases users actually file under #485 — "this
repo wants a different sandbox / approval policy / MCP server set / hard
sub-agent cap" — weren't covered.

### What ships

Adds the following keys to the project overlay, all merged with
identical "non-empty wins" semantics for strings:

- `provider` — pick a different backend per repo (e.g. `nvidia-nim` for
  an enterprise repo, `deepseek-cn` for a CN-team repo).
- `approval_policy` — `never` / `on-request` / `untrusted` for repos
  with strict policies.
- `sandbox_mode` — `read-only` / `workspace-write` / `danger-full-access`.
- `mcp_config_path` — per-repo MCP server set without touching the
  user's global file.
- `notes_path` — keep notes in-repo for projects where the notes tool
  is part of the dev workflow.

Plus two non-string fields:

- `max_subagents` (positive integer; clamped to `1..=MAX_SUBAGENTS=20`).
- `allow_shell` (bool).

### What stays user-global

`skills_dir`, `hooks`, `[capacity]`, `[retry]`, `[memory]`, etc. — those
are user-shaped settings, not repo-shaped. If a future use case
demands per-project values for any of them, a follow-up PR can extend
the overlay rather than letting the boundary blur.

### Tests

- 8 new tests in `project_config_tests` covering: provider+model,
  approval+sandbox, max_subagents+allow_shell, max_subagents
  clamping, negative-max_subagents rejection, missing config file
  pass-through, malformed TOML pass-through, and empty-string
  no-op.

### Docs

- New "Per-project overlay (#485)" section in `docs/CONFIGURATION.md`
  with a table of supported keys and the rationale for which fields
  stay user-global.

### Verification

cargo fmt --all -- --check                                          ✓
cargo clippy --workspace --all-targets --all-features --locked --   -D warnings   ✓
cargo test --workspace --all-features --locked                      ✓ 1828 + supporting (was 1820)

Closes #485

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/CONFIGURATION.md

@@ -18,6 +18,37 @@ Overrides:
 
 If both are set, `--config` wins. Environment variable overrides are applied after the file is loaded.
 
+### Per-project overlay (#485)
+
+When the TUI starts in a workspace that contains a
+`<workspace>/.deepseek/config.toml` file, the values declared in that
+file are merged on top of the global config. This lets a repo lock its
+own provider, model, sandbox policy, or approval policy without
+touching the user's `~/.deepseek/config.toml`. Pass
+`--no-project-config` to skip the overlay for one launch.
+
+Supported keys in the project overlay (top-level fields only):
+
+| Key | Effect |
+|---|---|
+| `provider` | switch backend (e.g. `"nvidia-nim"` for an enterprise repo) |
+| `model` | override `default_text_model` |
+| `api_key` | use a per-repo key (typically read from `.env`, **not committed**) |
+| `base_url` | point at a self-hosted endpoint |
+| `reasoning_effort` | force `"high"` / `"max"` for a complex repo |
+| `approval_policy` | `"never"` / `"on-request"` / `"untrusted"` for opinionated repos |
+| `sandbox_mode` | `"read-only"` / `"workspace-write"` / `"danger-full-access"` |
+| `mcp_config_path` | per-repo MCP server set |
+| `notes_path` | keep notes in-repo |
+| `max_subagents` | clamp concurrency for a constrained repo (clamped to 1..=20) |
+| `allow_shell` | gate shell tool access on `false` |
+
+The overlay is intentionally narrow — it covers the fields a repo
+maintainer is most likely to want to standardize across contributors.
+Other settings (skills_dir, hooks, capacity, retry, etc.) stay
+user-global. If your repo needs more, file an issue describing the
+specific use case.
+
 The `deepseek` facade and `deepseek-tui` binary share the same config file for
 DeepSeek auth and model defaults. `deepseek login --api-key ...` writes the
 root `api_key` field that `deepseek-tui` reads directly, and `deepseek --model